Low light level laser imaging system

ABSTRACT

A low light level imaging system having an image intensifier which utilizes a semiconductive material. A photoemissive material emits spatially modulated electrons in response to a detected image to spatially modulate the reflectivity of the semiconductive material. A laser beam from a laser located external to the image intensifier is reflected from the modulated semiconductor reflector to provide an image on a screen.

Unite Stimler States Patent [1 1 [451 May 22,1973

[54] LOW LIGHT LEVEL LASER IMAGING SYSTEM [75] Inventor: Morton Stimler,Rockville, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

22 Filed: June 25,1970

211 App1.No.: 49,640

[52] US. Cl. ..250/213 VT, 313/91, 350/160 [51] Int. Cl. ..H0lj 31/50[58] Field of Search ..250/213 VT, 83 R,

250/83 H, 80; 313/91, 92; 350/66, 160; 23/230 LC; 178/7.7

[56] References Cited UNITED STATES PATENTS 3,142,760 7/1964 lams..250/213 VT 3,379,998 4/1968 Soules et al. ..331/94.5 3,499,157 3/1970Satake et a] ..250/213 VT 3,500,237 3/1970 Myers et a1 ..331/94.53,544,711 12/1970 DeBitetto ...350/l60 X 3,253,497 5/1966 Dreyer..313/91 X Primary ExaminerWalter Stolwein Attorney-J1. S. Sciascia andJ. A. Cooke ABSTRACT A low light level imaging system having an imageintensifier which utilizes a semiconductive material. A photoemissivematerial emits spatially modulated electrons in response to a detectedimage to spatially modulate the reflectivity of the semiconductivematerial. A laser beam from a laser located external to the imageintensifier is reflected from the modulated semiconductor reflector toprovide an image on a screen.

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PHASE 8 BACKGROUND OF THE INVENTION This invention relates generally tothe art of image intensification and, more particularly, to a low lightlevel laser imaging system.

Image intensification has been widely used in both military and civilianapplications varying from target detection to crime prevention.Heretofore employed image intensifiers generally have included anoptical device that focuses a low intensity target image, or the like,on a layer of photoemissive material. Electrons released by thephotoemissive material in response to the impinging light imageprojected thereon are spatially modulated in the form of the lowintensity target image. These electrons are accelerated through a highpotential field, focused by an electron lens, and impinge on aluminescent screen, such as a phosphor or the like, to provide anintensified target image.

These heretofore employed conventional image intensifier devicesutilizing phosphor luminescent screens are, however, unsatisfactory insome respects. For example, increases in the sensitivity of such devicesprovided by increasing the electron accelerating potential are limitedby secondary electron emissions from the luminescent screen materialwhich cause the intensified image to lose its contrast and becomeblurred. Another limitation is the undesirable image persistence ofavailable phosphorous screens which renders the observation and trackingof moving targets somewhat difficult. A still further disadvantage ofconventional image intensifiers utilizing phosphorous screens is thatthe phosphor screens contain toxic materials which necessitate extremecare in assembly of the device and present a safety hazard in case ofbreakage thereof.

An image intensifier which converts a target image at low light to anintensified image without using a phosphorous or other luminescentscreen is disclosed in US. Patent application Ser. No. 889,445, filedDec. 3 l 1969, by Morton Stimler. A modified conventional electron imageintensifier is disclosed therein which utilizes a spatially modulatedsemiconductor material in lieu of the phosphor screen. The semiconductorpossesses variable or modulatable reflectivity and is utilized as an endreflector of a conjugate resonator laser cavity. The change inreflectivity of the semiconductor material responsive to the targetimage thereon causes the cavity to spatially laze to provide a targetimage at the laser output. If the thickness of the semiconductormaterial is specifically chosen, the image will be intensified inrelation to the low level intensity of the sensed target.

The laser image intensifier disclosed in the heretofore identifiedpatent application is adequate for most applications and eliminates thedisadvantages of conventional phosphor screen image intensifiersdiscussed hereinbefore. There are, however, some features of theintensifier which may render it unsuitable for some applications. Forexample, since the intensifier disclosed in the heretofore identifiedpatent application includes a conjugate resonator laser cavity internalto the intensifier system, the geometric configuration of the system issomewhat constrained. Similarly, since the laser is in a directalignment with the sensed target, the system is more prone to detectionby the target than if the laser were external to the intensifier and notin alignment with the target. Still furthermore, since the intensifieroperates by Q-switching which causes the laser to laze, the change inreflectivity of the semiconductor is quite critical, that is, areflectivity must be reached upon sensing a target which causes thelaser to laze. The thickness of the semiconductor material is alsohighly critical for providing proper image intensification.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toprovide a low light level laser imaging system.

Another object of the present invention is to provide a low light levellaser imaging system of greatly improved sensitivity.

Yet another object of the present invention is to provide a low lightlevel laser imaging system which is nontoxic.

Still another object of the instant invention is to provide a low lightlevel laser imaging system wherein the laser is external to the imageintensifier.

A further object of the invention is to provide a low light level laserimaging system which is free from constraining geometric configurations.

Another object of the instant invention is to provide a covert low lightlevel laser imaging system.

Briefly, these and other objects of the present invention are attainedby a low light level image intensifier utilizing a semiconductivematerial, Or the like, which is spatially modulated by a detected targetimage and varies in reflectivity in response thereto. A spatiallyfiltered and collimated source of light, such as provided by a laser,impinges on the semiconductor material to enhance the detected image.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of theinvention and many of the attendant advantages thereof will be readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 is a schematic view, partly in section, of the low light levellaser imaging system according to the present invention; and

FIGS. 2-4 are graphical diagrams of various parameters associated withthe system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingswherein like reference numerals designate identical or correspondingparts throughout the several views and, more particularly, to FIG. 1thereof wherein the laser imaging system according to the presentinvention for detecting a low light level target, or the like, shownschematically at 10 is shown as including a cylindrical housing 12enclosing an image intensifier 14 having at one extremity a targetimaging lens 16 for focusing light reflected from target 10 into theintensifier. It is readily apparent that imaging lens 16 may be replacedby a telescope or other suitable optical system adapted to focus a lightimage of the target into the image intensifier.

The optical image of target 10 is focused by lens 16 onto a conventionalfiber optics material or rods 18 disposed in housing 12 which transmitsthe focused image to a photoemissive screen or element 20 positioned inhousing 12 and having surfaces 22 and 24. Surface 22 of photoemissiveelement 20 abuts fiber optics material 18. it will be appreciated thatthe fiber optics may be eliminated and the target image may be focuseddirectly on the photoernissive material. Light photons, transferred byfiber optics 18 onto surface 22 in response to the impinging targetimage, causes electrons to be emitted from surface 24. The electrons soemitted by the photoernissive material are spatially modulated, that is,have a density which corresponds to the intensity of the target imagetransferred by the fiber optics. The spatially modulated electrons areaccelerated, through a high potential field V applied between electrodes26 and 27 located within housing 12, toward a focusing aperture 28formed centrally of electrode extensions 29 which form a conventionalelectron lens, and are projected, still spatially modulated, onto amodulatable element 30 located at the other extremity of housing 12. itis readily apparent that while the electron lens, formed by electrodeextensions 29, is funnellike in shape, other geometrically shapedelectron lenses may be utilized. Furthermore, while electrodes 26 and 27are shown as completely within housing 12, it is readily apparent thatthe electrodes may form a part of housing 12.

Modulatable element 30, rather than being formed of a phosphorous orluminescent material as used in conventional image intensifiers, iscomprised of a semiconductive material, such as, for example, leadtelluride, lead sulfide, lead selinide, or any equivalent material of athickness adapted to provide an increase in reflectivity in regions ofminority carrier injection. That is, the reflectivity of modulatablesemiconductive element 30 varies in direct proportion to the density ofelectrons impinging on a particular area of its surface. Thus, sincephotoernissive material emits greater quantities of electrons in areaswhere brighter light images are focused on it, the electrons so emittedare accelerated and focused on modulatable semiconductive elementcausing it to be more reflective in areas corresponding to brightertarget image areas. An image of target 10 is therefore formed onsemiconductive element 30 as a varied reflectivity pattern, the patternso formed retaining substantially the same shading and contrast as seenby imaging lens 16.

A conventional laser 32 is located external to image intensifier l4 andis adapted to supply light which is incident on semiconductor 30 at anangle 4). By positioning laser 32 external to the image intensifier andoff the longitudinal axis of housing 12, the possibility that the targetmay see the laser is greatly reduced and, therefore, a more covertimaging system is provided which is desirable if the system is used forsurveillance or other military applications. The light incident on thesemiconductor is spatially filtered and expanded by lenses 34 and 36 andby an aperture 38 to remove inhomogeneities in the laser beamcross-section which would otherwise be present. As hereinafter morefully explained, an expanded beam is advantageous if it is desired tovisually observe a target image 40 on a screen 42 or the like.

The low light level imaging system according to the present inventionmakes use of two characteristics of semiconductor 30 to provide anintensified target image 40 on screen 42. The first characteristic,heretofore mentioned, is that the reflectivity, which may be defined asthe ratio of reflected to incident intensity at the surface of amaterial only, is spatially modulated by the injection of electrons fromphotoernissive material 20. Thus, semiconductor material 30 will reflectlight incident thereon to a greater degree where the material isspatially modulated than elsewhere on the material. The secondcharacteristic is that of reflectance, which may be defined as the ratioof reflected to incident in tensity at the surface including multiplereflections from inner surface boundaries, which is an interferenceeffect. By utilizing changes in reflectance of semiconductor 30, as wellas changes in reflectivity of the semiconductor, a substantial imageamplification is attained.

More particularly, the semiconductor thickness d is so chosen so thatthe reflectance of the semiconductor ranges from a minima to a maxima tolight from laser 32 is incident on the semiconductor. Thus, thesemiconductor thickness d must be chosen to provide a change inreflectance from a minima to a maxima for radiation from laser 32.

When incident radiation of wavelength A, wherein A is the wavelength oflaser 32, strikes semiconductor 30, there is a phase change of 11'radians on reflections from the first surface and an additional phasechange due to the additional optical path length travelled by thatradiation which is reflected from the second surface. The additionaloptical path length, q, is given by the relatron:

wherein n is the index of refraction of the semiconductor, d is thesemiconductor thickness and is given by Snell s Law n sin'= sin where isthe angle the incident radiation makes with a normal to thesemiconductor surface outside the semiconductor as shown in FIG. 1. Thephase change, 8, in radians, due to the added path length q isillustrated in FIG. 2 as a function of reflectance. As indicatedtherein, minimum reflectance occurs at phase changes of 5 2p 11-(minima) p integer while maximum reflectance is given by the relation:

'6 (2p l)1r (maxima), p integer Relatively small index of refractionchanges which produce small changes in 6 and small changes inreflectivity, nonetheless, produce relatively large changes inreflectance of certain semiconductor materials.

The added path length requirement for minimum reflectance at incidentradiation A is given by the relation The change in optical path lengthbetween maximum and minimum reflectance is given by the relation q max qmin (2: A n)/cos wherein t is the electron penetration depth, A n is thechange in index of refraction. FIG. 3 illustrates a family of curveswhich shows electron penetration depth, 1, as 5 a function of density ofthe semiconductor with elec tron energy as a parameter. In general, theelectron energy and the density of semiconductor 30 will be known. Tofind the optimum thickness d, the electron penetration depth isdetermined from FIG. 3 and substituted for d in equation (5) is thenused to solve for p, which is an integer, and then this value of p isused to solve for d which is the optimum thickness that provides anoptimum change in reflectance, from a minima to a maxima, to incidentradiation of wavelength A.

To illustrate this procedure for a typical material, the calculationsfor a lead salt semiconductor, PbS, follows. Referring to FIG. 4 of thedrawing, a spectrometer plot of reflectance versus wavelength for a PbSfilm is shown. As shown therein, changes in reflectance of over 50percent may be obtained for wavelengths of laser 32 from about 3 toabout 6.5 microns. Consider an appropriate laser wavelength within thisrange of A 3.39;. and also assume cos z 1, that is, assume laser 32supplies radiation which is approximately perpendicular to semiconductor30. From any known text or the like on semiconductors, the density ofPbS is found to be 7.6 gm/cm" and the index of refraction for radiationof 3p. is n 4.10. Assuming an electron energy of 20 keV, which dependson the magnitude of the potential field applied between electrodes 26and 27 in FIG. 1 and may be predetermined, the electron penetration isfound from FIG. 3 to be about 3.7 1.. Substituting the electronpenetration depth for d in equation (5) and solving for p which must bean integer 35 yields.

Substituting p 9 into equation (5) and solving for d, which hadheretofore been estimated by electron penetration, shows that theoptimum thickness of semiconductor 30 is d 3.721.. This value ofsemiconductor thickness satisfies the minimum reflectance requirementwith a phase change of 8 18 1r. Additionally, since the thickness of thesemiconductor is approximately equal to the electron penetration depth,problems of absorptive losses within the semiconductor are The operationof the low light level imaging system may best be understood byreference to FIG. 1. As hereinbefore explained, a target 10 is sensed byimaging lens 16 and the image thereof is transferred to photoemissivematerial 20 via fiber optics 18. The photoemissive material responds tothe image by providing spatially modulated electrons which are emittedfrom the photoemissive material, accelerated by the potential betweenelectrodes 26 and 27 through focusing aperture 28 of the electron lens,and land incident to semiconductor 30. The electrons emitted fromphotoemissive material 20 spatially modulate the reflectivity ofsemiconductor 30 thereby providing an increased re flectivity andreflectance proportional to the target image.

Additionally, if the thickness d of the semiconductor is properlychosen, spatially filtered and expanded radiation of wavelength Aprovided by laser 32 incident to semiconductor 30 will be reflected inaccordance with the spatially modulated reflectance of the semiconductorto produce an image of the target 40 which may be viewed on screen 42.Thus, the effect of increased reflectivity and of increased reflectanceprovides an amplified or intensified target image.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. Thus, while thevarious thicknesses have been calculated for dz 0 corresponding to cos1b 1, it is readily apparent that a different angle of incidence may bechosen as desired. The new value of cos is then used in calculating theoptimized thickness. Similarly, it may be desirable to keep thethickness of the semiconductor relatively constant and obtain theoptimum reflectance change by varying the angle 4: thereby providinggreater flexibility in thickness requirement. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An imaging system comprising:

an image intensifier including a semiconductive material possessing avariable reflectivity and a variable reflectance and having thecharacteristics of increasing its reflectance in proportion to thedensity of electrons impinging upon it, and further includingphotoemissive means for spatially varying the reflectivity of saidmaterial in response to impingement of a detected image upon saidphotoemissive means, and

light means external to said image intensifier for providing anintensified image of said detected image proportionate to the reflectionof said material.

2. An imaging system according to claim 1 further comprising means foraccelerating said spatially emitted electrons, and

means for focusing said accelerated spatially emitted electrons ontosaid semiconductive material.

3. An imaging system according to claim 2 wherein said means external tosaid image intensifier for providing the image proportionate to thereflectance of said material is a means for providing radiation of apredetermined wavelength incident to said semiconductive material. v4.An imaging system according to claim 3 wherein the reflectance of saidsemiconductive material varies from a minima to a maxima.

5. An imaging system according to claim 4 wherein the variation ofreflectance of said semiconductive material is dependent upon thethickness of said semiconductive material.

6. An imaging system according to claim 5 wherein the thickness, d, ofsaid semiconductive material is given by the relation:

I of refraction of said semiconductive material.

d (p A Cos 7. An imaging system according to claim 3 wherein wherein pis an integer, A is said predetermined wavesaid means for providingradiation of a predeterlength, 4: is the angle said radiation makes witha normined wavelength is a laser. mal in said semiconductive material,and n is the index 5

1. An imaging system comprising: an image intensifier including asemiconductive material possessing a variable reflectivity and avariable reflectance and having the characteristics of increasing itsreflectance in proportion to the density of electrons impinging upon it,and further including photoemissive means for spatially varyinG thereflectivity of said material in response to impingement of a detectedimage upon said photoemissive means, and light means external to saidimage intensifier for providing an intensified image of said detectedimage proportionate to the reflection of said material.
 2. An imagingsystem according to claim 1 further comprising means for acceleratingsaid spatially emitted electrons, and means for focusing saidaccelerated spatially emitted electrons onto said semiconductivematerial.
 3. An imaging system according to claim 2 wherein said meansexternal to said image intensifier for providing the image proportionateto the reflectance of said material is a means for providing radiationof a predetermined wavelength incident to said semiconductive material.4. An imaging system according to claim 3 wherein the reflectance ofsaid semiconductive material varies from a minima to a maxima.
 5. Animaging system according to claim 4 wherein the variation of reflectanceof said semiconductive material is dependent upon the thickness of saidsemiconductive material.
 6. An imaging system according to claim 5wherein the thickness, d, of said semiconductive material is given bythe relation: d (p lambda cos phi '')/2n wherein p is an integer, lambdais said predetermined wavelength, phi '' is the angle said radiationmakes with a normal in said semiconductive material, and n is the indexof refraction of said semiconductive material.
 7. An imaging systemaccording to claim 3 wherein said means for providing radiation of apredetermined wavelength is a laser.